Not a daft question at all Athley, but the only birds that eat far more than they need are the avian vacuum cleaners - known as the Wood Pigeon, so don't worry unless your visitors can't get off the ground :-)
Birds aren't stupid and will always go for the easiest option, fill up, then make a quick getaway, and fat balls meet all these needs. Clearly, just eating these lumps of fat all the time is not a good thing, so if you think they are concentrating on these alone and not going to any of the other goodies on offer then don't put them out for a while as it won't do any harm, although not during bitterly cold weather.